The Present Condition and Future Prospects of the Active Fault Study in Japan

Abstract

This paper critically reviewed the recent trends of the active fault study in Japan from the geological and geomorphological view points, and presented the future prospects and directions to recover the vigor and hope of the study.
The active fault study in Japan having a progressive history of more than one hundred years has steadily accumulated various kind of data on the fault activity. In the end of 1970's, the Research Group for Active Fault systematically compiled these data. They published the first nationwide inventory of active fault titled “Active Faults in Japan” in 1980. This was the epoch-making result for the active fault research in Japan. Since then, the trend of the study has forked into following three courses. 1) The detailed reconstruction and description of the fault behavior, 2) The understanding of the relationship between active faults and regional or plate tectonics, and 3) The interpretation of the active fault information to the other scientific fields such as geophysics and civil engineering.
The first course of these was the main trend of the active fault study in 1980's. In this trend, the trench excavation survey appeared as a useful method to reconstruct the paleoseismicity and fault activity. More than 60 trench surveys during the last 15 years revealed following two important results on the fault behavior. One is that the almost all of onshore active faults in Japan have the recurrence interval longer than 1, 000 years. The other result is that the trenching could confirm the source faults of the huge historical earthquakes of which hypocenters were previously unknown.
Although the active fault study based on the trench excavations obtained thus fruitful results, on the other hand, some signs suggesting the declining of the study have appeared. These were the shortage of young scientist who interested with the active fault, and the decreasing of presentation numbers in the meeting. The declining of the study might be caused by the loss of attractiveness for young scientist due to the over-depending on the trench survey.
However, the earthquake is a geoglogical phenomenon as well as a physical phenomenon. The active fault is an indispensable element to recognize the essence of earthquake. Therefore the author proposes the following three major items that is necessary for our study to overcome the recently rising problems.
1) To distinguish the segment barriers from the major active fault systems in Japan ; The barrier in the segmented fault system has a potential to be a rupture nucleation. Recognition of the barrier is very important for improvement of continual observatory system for precursor. Therefore we need much information on the paleoseismicity and activity of each discrete fault constituting a fault system. It is necessary to invent the various methods for the finding of segmented structure as well as the improvement of trench method.
2) To establish the seismotectonic model ; Seismotectonic model implies a kind of model that explains the relation between the earthquake, the active fault and the regional or plate tectonics. Ishibashi (1988) constructed the excellent one to show the plate tectonic implication for the future probable earthquake, called Odawara earthquake, in the northwestern part of Sagami Bay. The author thinks that we should build this type of seismotectonic model not only in Sagami Bay but also in various parts of Japan. The complement of the seismotectonic model in many parts of Japan will refine the ability of future prediction of large earthquake caused by onshore active fault.